split, fix and cleanup predef and runBefore handling

[mpollmeier]

Prior to 0.1.92 we concatenated and executed all predef code. That was very simple but had quite
a few downsides, including incorrect line number reporting and some edge cases when handling complex
compilation units, e.g. ordering of imports etc.

#157 changed this behaviour to instead compile
the given files and add the results to the classpath. At the same time we unintendedly broke
the ability to execute code on startup of the repl - that's reintroduced as a new parameter
--runBefore now.